1. Field of the Invention
This invention relates to a process for forming sheet metal stock, by engaging an unheated sheet metal workpiece between a concave die and a mating convex punch which are caused to move relative to one another, thereby drawing the workpiece into the internal configuration of the die. The process has utility in the drawing of hot rolled or cold rolled sheet metal stock whose yield strength and tensile strength are sensitive to small changes in temperature during a drawing operation, particularly under ambient conditions.
2. Description of the Prior Art
In extrusion type metal forming operations, wherein compression is applied on the metal being formed, it is customary to heat the workpiece to a relatively high temperature in order to reduce the press load. In such operations, the provision of means for cooling or chilling parts of the extrusion apparatus, particularly the punch, to prevent thermal distortion and/or excessive wear, is known in the art, as exemplified in U.S. Pat. Nos. 3,740,990 to Prajsnar et al. 3,200,625 to Wehmeyer and 3,808,865 to Wagner et al.
U.S. Pat. No. 3,196,648 issued to Molnar, discloses an impact extrusion process wherein the punch is provided with outwardly extending passages which exit from the wall of the punch behind the head thereof, through which pressurized gas is caused to flow during the forming operation. It is stated that this process results in elimination of frictional restraint between the punch, die and the part being formed during the extrusion and stripping. Rapid heat transfer results in cooling of the metal in the formed walls, thereby increasing the strength thereof sufficiently to support loads imposed by friction.
The prior art has thus disclosed the cooling of the punch or other parts in a hot impact extrusion process.
Sheet metal forming is not analagous to hot extrusion. In an extrusion process the forming stresses on a heated workpiece are compressive, so that it cannot fail from the softening imparted by the heating. The only limit on the amount of heat which can be used is the increased wear on the tooling and possible thermal distortion. On the other hand, in sheet metal forming, the stresses imposed on the workpiece, e.g., during a simple cup-forming operation, include tension in the side wall, straightening, and bending, in different areas. With the exception of a small group of brittle alloys which must be heated above their recrystallization temperature to become ductile, the workpiece is never heated since the walls must support the entire forming load. Heating would not only reduce wall strength, but would also decrease the effectiveness of the lubrication used to reduce friction. On the other hand, some heat is unavoidably generated by the plastic deformation in the sheet metal workpiece and by friction between the workpiece and the die. Much of this heat is absorbed by the relatively massive tooling.
In the field of sheet metal forming and drawing, a book entitled "Deformation Processing" (published in 1972) refers at pages 236 and 237 to the possibility of raising the drawing limit by strengthening "the potential failure site. . . relative to the deformation zone where the load originates." There are mechanical and metallurgical possibilities. Metallurgically, the possibilities relate "to the controlled yield-locus distortion." It is pointed out the deformation in the flange is pure shear while deformation around the punch is essentially plane strain with each being related to the yield-locus. To raise the drawing limit "the failure site should be strengthened by increasing its path length through quadrant I relative to that of the flange through quadrant IV (FIG. 11-9)." It is then stated:
"An obvious possibility is a temperature drop, from flange to cup wall. Another is an increase in the anisotropy parameter, B,. . . or the more usual R. . . which are both connected to crystallographic texture. . . "
French Pat. No. 1,506,899 relates to the hot drawing of alloy sheet material (such as aluminum, zinc and/or magnesium alloys) which must be heated above the recrystallization temperature (175.degree.-315.degree. C.) in order to be drawn. In the process of this patent, the punch of the press is cooled while the die is kept at the hot drawing temperature by heating. This concept is thus similar to that of the previously mentioned U.S. Pat. No. 3,196,648, although applied to drawing rather than impact extrusion.
U.S. Pat. No. 2,396,218 discloses the drawing of a magnesium alloy sheet material which is heated to a temperature between 400.degree. and 700.degree. F., wherein the punch is cooled. This disclosure is quite similar to that of the above-mentioned French patent.
U.S. Pat. No. 3,577,753 discloses a drawing method and apparatus wherein an internally fluid-cooled punch and fluid-cooled dies are provided in order to prevent thermal break-down of dry film lubricant coating on the sheet metal workpiece.
The mechanism of draw forming cold rolled, low carbon steel sheet is still not fully understood, and problems exist in the deep drawing of such sheet material, despite recent advances in composition and processing which provide a more favorable grain texture. The concept of heating of the workpieces, as taught in the above French Pat. No. 1,506,899 and U.S. Pat. No. 2,396,218, while effective for hard and brittle sheet material such as aluminum-magnesium alloys and titanium, has been found to decrease the formability of ductile and malleable sheet material such as cold rolled low carbon steel. Heating results in break-down of conventional drawing lubricants with a resulting danger of scoring of the drawn parts. Since the surface appearance of drawn sheet metal parts is usually critical, scoring, pinching and oxide scaling cannot be tolerated. Cooling of all tooling as taught in U.S. Pat. No. 3,577,753, increases the strength of the workpiece in the areas which must be plastically deformed. This increases the load on the sidewalls of the part being drawn, which leads to failure.